To leverage the smart routing options in the NOC Auth-DNS service a zone file must have more than one A record for the same domain.
From the Authoritative DNS dashboard, click the name of the domain in your domain list.
This will automatically open the zone file for the domain.
Click Add New Record
Select the A record type.
Name your subdomain, in our example we'll go with smart.
Repeat the process twice, using the same name for the subdomain, but selecting a different IP for the origin.
Doing this will automatically activate the enhanced records interface.
Click on edit enhanced options to access the smart routing options: